title: Abundant type III lipid transfer proteins in Arabidopsis tapetum are secreted to the locule and become a constituent of the pollen exine.
abstract: Lipid transfer proteins (LTPs) are small secretory proteins in plants with defined lipid-binding structures for possible lipid exocytosis. Special groups of LTPs unique to the anther tapetum are abundant, but their functions are unclear. We studied a special group of LTPs, type III LTPs, in Arabidopsis (Arabidopsis thaliana). Their transcripts were restricted to the anther tapetum, with levels peaking at the developmental stage of maximal pollen-wall exine synthesis. We constructed an LTP-Green Fluorescent Protein (LTP-GFP) plasmid, transformed it into wild-type plants, and monitored LTP-GFP in developing anthers with confocal laser scanning microscopy. LTP-GFP appeared in the tapetum and was secreted via the endoplasmic reticulum-trans-Golgi network machinery into the locule. It then moved to the microspore surface and remained as a component of exine. Immuno-transmission electron microscopy of native LTP in anthers confirmed the LTP-GFP observations. The in vivo association of LTP-GFP and exine in anthers was not observed with non-type III or structurally modified type III LTPs or in transformed exine-defective mutant plants. RNA interference knockdown of individual type III LTPs produced no observable mutant phenotypes. RNA interference knockdown of two type III LTPs produced microscopy-observable morphologic changes in the intine underneath the exine (presumably as a consequence of changes in the exine not observed by transmission electron microscopy) and pollen susceptible to dehydration damage. Overall, we reveal a novel transfer pathway of LTPs in which LTPs bound or nonbound to exine precursors are secreted from the tapetum to become microspore exine constituents; this pathway explains the need for plentiful LTPs to incorporate into the abundant exine.
authors.affiliation: Institute of Plant and Microbial Biology, Academia Sinica, Taipei, Taiwan 11529.
authors.forename: Ming-Der
authors.initials: MD
authors.lastname: Huang
authors.forename: Tung-Ling L
authors.initials: TL
authors.lastname: Chen
authors.forename: Anthony H C
authors.initials: AH
authors.lastname: Huang
mesh_terms: D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D002352:Carrier Proteins / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism; D035264:Flowers / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure; D018506:Gene Expression Regulation, Plant; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D018613:Microscopy, Confocal; D008855:Microscopy, Electron, Scanning; D046529:Microscopy, Electron, Transmission; D016253:Microscopy, Immunoelectron; D010802:Phylogeny; D030821:Plants, Genetically Modified; D011058:Pollen / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure; D021381:Protein Transport; D034622:RNA Interference; D020133:Reverse Transcriptase Polymerase Chain Reaction; D021601:trans-Golgi Network / Q000378:metabolism
chemical_list: D029681:Arabidopsis Proteins; D002352:Carrier Proteins; C030363:lipid transfer protein; D049452:Green Fluorescent Proteins